CS162 Operating Systems and Systems Programming Lecture 7 Readers-Writers Language Support for Synchronization February 13, 2008 Prof. Anthony D. Joseph

CS162Operating Systems andSystems Programming

Lecture 7

Readers-WritersLanguage Support for

Synchronization

February 13, 2008

Prof. Anthony D. Joseph

http://inst.eecs.berkeley.edu/~cs162

Lec 7.22/13/08 Joseph CS162 ©UCB Spring 2008

Review: Semaphores

• Definition: a Semaphore has a non-negative integer value and supports the following two operations:– Only time can set integer directly is at

initialization time

• P(): an atomic operation that waits for semaphore to become positive, then decrements it by 1 – Think of this as the wait() operation

• V(): an atomic operation that increments the semaphore by 1, waking up a waiting P, if any– This of this as the signal() operation


Review: Full Solution to Bounded Buffer

Semaphore fullBuffer = 0; // Initially, no cokeSemaphore emptyBuffers = numBuffers;

// Initially, num empty slotsSemaphore mutex = 1; // No one using machine

Producer(item) {emptyBuffers.P(); // Wait until spacemutex.P(); // Wait until buffer freeEnqueue(item);mutex.V();fullBuffers.V(); // Tell consumers there is

// more coke}Consumer() {

fullBuffers.P(); // Check if there’s a cokemutex.P(); // Wait until machine freeitem = Dequeue();mutex.V();emptyBuffers.V(); // tell producer need morereturn item;

}


Review: Discussion about Solution

• Is order of P’s important? – Yes! Can cause deadlock

• Is order of V’s important? – No, except for scheduling efficiency

• What if we have 2 producers or 2 consumers?– Nothing changes!

• Semaphores are a huge step up, but:– They are confusing because they are dual purpose:

» Both mutual exclusion and scheduling constraints» Example: the fact that flipping of P’s in bounded buffer

gives deadlock is not immediately obvious– Cleaner idea: Use locks for mutual exclusion and

condition variables for scheduling constraints


Review: Monitors and Condition Variables

• Definition: Monitor: a lock and zero or more condition variables for managing concurrent access to shared data– Use of Monitors is a programming paradigm– Some languages like Java provide monitors in

the language

• The lock provides mutual exclusion to shared data:– Always acquire before accessing shared data

structure– Always release after finishing with shared data– Lock initially free


Goals for Today

• Continue with Synchronization Abstractions– Monitors and condition variables

• Readers-Writers problem and solution• Language Support for Synchronization• An Overview of ACID properties in a DBMS

Note: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne

Note: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne. Many slides generated from my lecture notes by Kubiatowicz.


Simple Monitor Example (version 1)

• Here is an (infinite) synchronized queueLock lock;Queue queue;

AddToQueue(item) {lock.Acquire(); // Lock shared dataqueue.enqueue(item); // Add itemlock.Release(); // Release Lock

}

RemoveFromQueue() {lock.Acquire(); // Lock shared dataitem = queue.dequeue();// Get next item or

nulllock.Release(); // Release Lockreturn(item); // Might return null

}


Condition Variables• How do we change the RemoveFromQueue()

routine to wait until something is on the queue?– Could do this by keeping a count of the number of

things on the queue (with semaphores), but error prone

• Condition Variable: a queue of threads waiting for something inside a critical section– Key idea: allow sleeping inside critical section by

atomically releasing lock at time we go to sleep– Contrast to semaphores: Can’t wait inside critical

section• Operations:

– Wait(&lock): Atomically release lock and go to sleep. Re-acquire lock later, before returning.

– Signal(): Wake up one waiter, if any– Broadcast(): Wake up all waiters

• Rule: Must hold lock when doing condition variable ops!– In Birrell paper, he says can perform signal() outside

of lock – IGNORE HIM (this is only an optimization)


Complete Monitor Example (with condition variable)

• Here is an (infinite) synchronized queueLock lock;Condition dataready;Queue queue;

AddToQueue(item) {lock.Acquire(); // Get Lockqueue.enqueue(item); // Add itemdataready.signal(); // Signal any

waiterslock.Release(); // Release Lock

}

RemoveFromQueue() {lock.Acquire(); // Get Lockwhile (queue.isEmpty()) {

dataready.wait(&lock); // If nothing, sleep}item = queue.dequeue(); // Get next itemlock.Release(); // Release Lockreturn(item);

}


Mesa vs. Hoare monitors• Need to be careful about precise definition of

signal and wait. Consider a piece of our dequeue code:

while (queue.isEmpty()) {dataready.wait(&lock); // If nothing, sleep}item = queue.dequeue(); // Get next item

– Why didn’t we do this?if (queue.isEmpty()) {dataready.wait(&lock); // If nothing, sleep}item = queue.dequeue(); // Get next item

• Answer: depends on the type of scheduling– Hoare-style (most textbooks):

» Signaler gives lock, CPU to waiter; waiter runs immediately

» Waiter gives up lock, processor back to signaler when it exits critical section or if it waits again

– Mesa-style (Nachos, most real operating systems):» Signaler keeps lock and processor» Waiter placed on ready queue with no special priority» Practically, need to check condition again after wait


Administrivia

• First design document due tomorrow by 11:59pm– Use the newsgroup for questions (search Google

groups)• Design reviews:

– Everyone must attend! (no exceptions)– 2 points off for one missing person– 1 additional point off for each additional missing person– Penalty for arriving late (arrive 5—10 mins early)– Sign up link to be posted pages

• What we expect in document/review:– Architecture, correctness constraints, algorithms,

pseudocode, NO CODE!– Important: testing strategy, and test case types

• Midterm I in two weeks: Wed 2/27, TBA 6-7:30– Closed book, no notes, no calculators/PDAs– Topics: Everything up to 2/20 (Sunday 2/24 review

session)


Readers/Writers Problem

• Motivation: Consider a shared database– Two classes of users:

» Readers – never modify database» Writers – read and modify database

– Is using a single lock on the whole database sufficient?» Like to have many readers at the same time» Only one writer at a time

RR

R

W


Basic Readers/Writers Solution• Correctness Constraints:

– Readers can access database when no writers– Writers can access database when no readers or

writers– Only one thread manipulates state variables at a

time• Basic structure of a solution:

– Reader() Wait until no writers Access data base Check out – wake up a waiting writer

– Writer() Wait until no active readers or writers Access database Check out – wake up waiting readers or writer

– State variables (Protected by a lock called “lock”):» int AR: Number of active readers; initially = 0» int WR: Number of waiting readers; initially = 0» int AW: Number of active writers; initially = 0» int WW: Number of waiting writers; initially = 0» Condition okToRead = NIL» Conditioin okToWrite = NIL


Code for a Reader

Reader() {// First check self into systemlock.Acquire();

while ((AW + WW) > 0) { // Is it safe to read?WR++; // No. Writers existokToRead.wait(&lock); // Sleep on cond varWR--; // No longer waiting

}

AR++; // Now we are active!lock.release();

// Perform actual read-only accessAccessDatabase(ReadOnly);

// Now, check out of systemlock.Acquire();AR--; // No longer activeif (AR == 0 && WW > 0) // No other active

readersokToWrite.signal(); // Wake up one writer

lock.Release();}

Why Release the Lock here?


Writer() {// First check self into systemlock.Acquire();while ((AW + AR) > 0) { // Is it safe to write?

WW++; // No. Active users existokToWrite.wait(&lock); // Sleep on cond varWW--; // No longer waiting

}AW++; // Now we are active!lock.release();// Perform actual read/write accessAccessDatabase(ReadWrite);// Now, check out of systemlock.Acquire();AW--; // No longer activeif (WW > 0){ // Give priority to writers

okToWrite.signal(); // Wake up one writer} else if (WR > 0) { // Otherwise, wake reader

okToRead.broadcast(); // Wake all readers}lock.Release();

}

Why Give priority to writers?

Code for a Writer

Why broadcast()

here instead of signal()?


Simulation of Readers/Writers solution

• Consider the following sequence of operators:– R1, R2, W1, R3

• On entry, each reader checks the following:while ((AW + WW) > 0) { // Is it safe to read?

WR++; // No. Writers existokToRead.wait(&lock); // Sleep on cond varWR--; // No longer waiting

}

AR++; // Now we are active!

• First, R1 comes along:AR = 1, WR = 0, AW = 0, WW = 0

• Next, R2 comes along:AR = 2, WR = 0, AW = 0, WW = 0

• Now, readers make take a while to access database– Situation: Locks released– Only AR is non-zero


Simulation(2)

• Next, W1 comes along:while ((AW + AR) > 0) { // Is it safe to write?

WW++; // No. Active users exist

okToWrite.wait(&lock); // Sleep on cond varWW--; // No longer waiting

}AW++;

• Can’t start because of readers, so go to sleep:AR = 2, WR = 0, AW = 0, WW = 1

• Finally, R3 comes along:AR = 2, WR = 1, AW = 0, WW = 1

• Now, say that R2 finishes before R1:AR = 1, WR = 1, AW = 0, WW = 1

• Finally, last of first two readers (R1) finishes and wakes up writer:

if (AR == 0 && WW > 0) // No other active readers

okToWrite.signal(); // Wake up one writer


Simulation(3)

• When writer wakes up, get:AR = 0, WR = 1, AW = 1, WW = 0

• Then, when writer finishes:if (WW > 0){ // Give priority to

writersokToWrite.signal(); // Wake up one writer

} else if (WR > 0) { // Otherwise, wake reader

okToRead.broadcast(); // Wake all readers}

– Writer wakes up reader, so get:AR = 1, WR = 0, AW = 0, WW = 0

• When reader completes, we are finished


Questions• Can readers starve? Consider Reader() entry code:

while ((AW + WW) > 0) { // Is it safe to read?WR++; // No. Writers existokToRead.wait(&lock); // Sleep on cond varWR--; // No longer waiting

}AR++; // Now we are active!

• What if we erase the condition check in Reader exit?AR--; // No longer activeif (AR == 0 && WW > 0) // No other active readers

okToWrite.signal(); // Wake up one writer

• Further, what if we turn the signal() into broadcast()AR--; // No longer activeokToWrite.broadcast(); // Wake up one writer

• Finally, what if we use only one condition variable (call it “okToContinue”) instead of two separate ones?– Both readers and writers sleep on this variable– Must use broadcast() instead of signal()


Can we construct Monitors from Semaphores?

• Locking aspect is easy: Just use a mutex• Can we implement condition variables this way?

Wait() { semaphore.P(); }Signal() { semaphore.V(); }

– Doesn’t work: Wait() may sleep with lock held• Does this work better?

Wait(Lock lock) { lock.Release(); semaphore.P(); lock.Acquire();}Signal() { semaphore.V(); }

– No: Condition vars have no history, semaphores have history:» What if thread signals and no one is waiting? NO-OP» What if thread later waits? Thread Waits» What if thread V’s and noone is waiting? Increment» What if thread later does P? Decrement and

continue


Construction of Monitors from Semaphores (con’t)

• Problem with previous try:– P and V are commutative – result is the same no

matter what order they occur– Condition variables are NOT commutative

• Does this fix the problem?Wait(Lock lock) { lock.Release(); semaphore.P(); lock.Acquire();}Signal() { if semaphore queue is not empty semaphore.V();}

– Not legal to look at contents of semaphore queue– There is a race condition – signaler can slip in after

lock release and before waiter executes semaphore.P()

• It is actually possible to do this correctly– Complex solution for Hoare scheduling in book– Can you come up with simpler Mesa-scheduled

solution?


Monitor Conclusion

• Monitors represent the logic of the program– Wait if necessary– Signal when change something so any waiting

threads can proceed• Basic structure of monitor-based program:

lock while (need to wait) { condvar.wait();}unlock

do something so no need to wait

lock

condvar.signal();

unlock

Check and/or updatestate variables

Wait if necessary

Check and/or updatestate variables

BREAK


C-Language Support for Synchronization

• C language: Pretty straightforward synchronization– Just make sure you know all the code paths

out of a critical sectionint Rtn() {

lock.acquire();…if (exception) {

lock.release();return errReturnCode;

}…lock.release();return OK;

}– Watch out for setjmp/longjmp!

» Can cause a non-local jump out of procedure» In example, procedure E calls longjmp, poping

stack back to procedure B» If Procedure C had lock.acquire, problem!

Proc A

Proc BCalls setjmp

Proc Clock.acquire

Proc D

Proc ECalls longjmp

Sta

ck g

row

th


C++ Language Support for Synchronization

• Languages with exceptions like C++– Languages that support exceptions are

problematic (easy to make a non-local exit without releasing lock)

– Consider:void Rtn() {

lock.acquire();…DoFoo();…lock.release();

}void DoFoo() {

…if (exception) throw errException;…

}– Notice that an exception in DoFoo() will exit

without releasing the lock


C++ Language Support for Synchronization (con’t)• Must catch all exceptions in critical sections

– Catch exceptions, release lock, and re-throw exception:

void Rtn() {lock.acquire();try {

…DoFoo();…

} catch (…) { // catch exceptionlock.release(); // release lockthrow; // re-throw the

exception}lock.release();

}void DoFoo() {

…if (exception) throw errException;…

}– Even Better: auto_ptr<T> facility. See C++ Spec.

» Can deallocate/free lock regardless of exit method


Java Language Support for Synchronization

• Java has explicit support for threads and thread synchronization

• Bank Account example:class Account {

private int balance;// object constructorpublic Account (int initialBalance) {

balance = initialBalance;}public synchronized int getBalance() {

return balance;}public synchronized void deposit(int amount)

{balance += amount;

}}

– Every object has an associated lock which gets automatically acquired and released on entry and exit from a synchronized method.


Java Language Support for Synchronization (con’t)

• Java also has synchronized statements:synchronized (object) {

…}

– Since every Java object has an associated lock, this type of statement acquires and releases the object’s lock on entry and exit of the body

– Works properly even with exceptions:synchronized (object) {

…DoFoo();…

}void DoFoo() {

throw errException;}


Java Language Support for Synchronization (con’t 2)

• In addition to a lock, every object has a single condition variable associated with it– How to wait inside a synchronization method or

block:» void wait(long timeout); // Wait for timeout» void wait(long timeout, int nanoseconds); //variant» void wait();

– How to signal in a synchronized method or block:» void notify(); // wakes up oldest waiter» void notifyAll(); // like broadcast, wakes everyone

– Condition variables can wait for a bounded length of time. This is useful for handling exception cases:

t1 = time.now();while (!ATMRequest()) {

wait (CHECKPERIOD);t2 = time.new();if (t2 – t1 > LONG_TIME) checkMachine();

}– Not all Java VMs equivalent!

» Different scheduling policies, not necessarily preemptive!


ACID

• How does a database handle concurrency?– It provides A.C.I.D. properties – Atomicity,

Consistency, Isolation, Durability

• Key concept: Transaction– An atomic sequence of database actions

(reads/writes)» Actions all happen or none at all

– Takes DB from one consistent state to another

consistent state 1 consistent state 2transaction


DBMS Consistency Example

• Here, consistency is based on our knowledge of banking “semantics”

• In general, up to writer of transaction to ensure transaction preserves consistency

• DBMS provides (limited) automatic enforcement, via integrity constraints– e.g., balances must be >= 0

checking: $200savings: $1000

transaction checking: $300savings: $900


Challenge: Concurrent transactions• Goal: execute xacts {T1, T2, … Tn}, and ensure a

consistent outcome– Isolate xact’s intermediate state from other xacts

• One option: “serial” schedule (one after another)

• Better: allow interleaving of xact actions, as long as outcome is equivalent to some serial schedule

• Two possible enforcement methods– Optimistic: permit arbitrary interleaving, then

check equivalence to serial schedule– Pessimistic: xacts set locks on data objects, such

that illegal interleaving is impossible» More on locking in another lecture…


Ensuring Transaction Properties

• DBMS ensures: – Atomicity even if xact aborted (due to deadlock, system

crash, …)– Durability (persistence) of committed xacts, even if system

crashes

• Idea: Keep a log of all actions carried out by the DBMS:– Record all DB modifications in log, before they are executed– To abort a xact, undo logged actions in reverse order– If system crashes, must:

1) undo partially executed xacts (ensures atomicity)2) redo committed xacts (ensures durability)

– Much trickier than it sounds!


• Atomicity: guarantee that either all of the tasks of a transaction are performed, or none of them are

• Consistency: database is in a legal state when the transaction begins and when it ends – a transaction cannot break the rules, or integrity constraints

• Isolation: operations inside a transaction appear isolated from all other operations – no operation outside transaction can see data in an intermediate state

• Durability: guarantee that once the user has been notified of success, the transaction will persist (survive system failure)

ACID Summary


Summary• Monitors: A lock plus one or more condition

variables– Always acquire lock before accessing shared

data– Use condition variables to wait inside critical

section» Three Operations: Wait(), Signal(), and Broadcast()

• Readers/Writers– Readers can access database when no writers– Writers can access database when no readers– Only one thread manipulates state variables at a

time

• Language support for synchronization:– Java provides synchronized keyword and one

condition-variable per object (with wait() and notify())

Documents

CS162 Operating Systems and Systems Programming Lecture 7 Readers-Writers Language Support for Synchronization February 13, 2008 Prof. Anthony D. Joseph